In the semiconductor manufacturing industry, photoresist materials are used for transferring an image to one or more underlying layers, such as metal, semiconductor and dielectric layers, disposed on a semiconductor substrate, as well as to the substrate itself. To increase the integration density of semiconductor devices and allow for the formation of structures having dimensions in the nanometer range, photoresists and photolithography processing tools having high-resolution capabilities have been and continue to be developed.
Positive-tone chemically amplified photoresists are conventionally used for high-resolution processing. Such resists typically employ a resin having acid-labile leaving groups and a photoacid generator. Exposure to actinic radiation causes the acid generator to form an acid which, during post-exposure baking, causes cleavage of the acid-labile groups in the resin. This creates a difference in solubility characteristics between exposed and unexposed regions of the resist in an aqueous alkaline developer solution. Exposed regions of the resist are soluble in the aqueous alkaline developer and are removed from the substrate surface, whereas unexposed regions, which are insoluble in the developer, remain after development to form a positive image.
One approach to achieving nm-scale feature sizes in semiconductor devices is the use of short wavelengths of light, for example, 193 nm or less, during exposure of chemically amplified photoresists. To further improve lithographic performance, immersion lithography tools have been developed to effectively increase the numerical aperture (NA) of the lens of the imaging device, for example, a scanner having a KrF or ArF light source. This is accomplished by use of a relatively high refractive index fluid (i.e., an immersion fluid) between the last surface of the imaging device and the upper surface of the semiconductor wafer. The immersion fluid allows a greater amount of light to be focused into the resist layer than would occur with an air or inert gas medium. When using water as the immersion fluid, the maximum numerical aperture can be increased, for example, from 1.2 to 1.35. With such an increase in numerical aperture, it is possible to achieve a 40 nm half-pitch resolution in a single exposure process, thus allowing for improved design shrink. This standard immersion lithography process, however, is generally not suitable for manufacture of devices requiring greater resolution, for example, for the 32 nm and 22 nm half-pitch nodes.
In an effort to achieve greater resolution and to extend capabilities of existing manufacturing tools, advanced patterning techniques such as various double patterning processes (also referred to as pitch splitting) have been proposed. Another advanced patterning technique for obtaining fine lithographic patterns involves negative tone development (NTD) of traditionally positive-type chemically amplified photoresist materials. In negative tone development, a negative image can be obtained from a traditionally positive-type resist by development with particular organic solvents. Such a process is described, for example, in U.S. Pat. No. 6,790,579 to Goodall et al. That document discloses a photoresist composition comprising an acid-generating initiator and a polycyclic polymer containing recurring acid labile pendant groups along the polymer backbone. The exposed areas can be selectively removed with an alkaline developer or, alternatively, the unexposed regions can be selectively removed by treatment with a suitable nonpolar solvent for negative tone development.
U.S. Application Publication No. 2009/0042147A1 to Tsubaki et al also discloses a negative tone development process. In the described process, a substrate is coated with a resist composition, a protective film containing a resin having a silicon or fluorine atom is formed on the resist composition, and the resist film is exposed via an immersion medium and developed with a negative developer. That document discloses various organic developers and developer mixtures. Among the developer materials described in the examples of this document, butyl acetate is commonly used. The present inventors have found that acceptable patterns can be imaged for larger feature sizes using n-butyl acetate as the developer. With reductions in feature sizes and increases in aspect ratios, however, it becomes difficult to obtain accurate resist patterns using this developer. For example, it has been found that when forming smaller contact holes, n-butyl acetate tends to result in missing contact holes with increased exposure energy. This tendency has been found to become exacerbated with resist formulations having higher molecular weight polymers that exhibit poor solubility in n-butyl acetate. It has further been found during studies for the present invention that other developer solvents such as 2-heptanone, while providing excellent imaging at greater resolutions, exhibit a relatively slow exposure window with significant thickness loss after development for memory applications.
There is a continuing need in the art for developer compositions and photolithographic patterning processes which address one or more of the problems associated with the state of the art and which allow for the formation of fine patterns in electronic device fabrication. It has been found that use of particular combinations of organic developers can provide for excellent lithographic performance in negative tone development processes. Quite surprisingly, it has been discovered that developer mixtures of the invention can exhibit positive attributes of while avoiding or minimizing negative characteristics of the individual developers.